Here's the history:
#2 Israel is on the southern border
1975 In April, civil war erupted again in Lebanon (see also
1958) when Christian members of the "Phalange" militia
in Lebanon ambushed a bus carrying Palestinians from
the refugee camps of Sabra and Shatilla (see below) to the
Tel al-Zataar refugee camp (see massacre at Tel al-
Zataar) killing 30 people.
The PLO retaliated.
Lebanon's Christians had grown steadily uneasy
with challenges to their political supremacy (based on the
1943 National Pact) coming from Lebanon's Sunni, Shia,
and Druze
populations.
In July, 1976, the Syrian army entered Lebanon and imposed
a ceasefire. Syria involved itself initially to protect Christians
from defeat at the hands of the
Muslims.
On March 15, 1978, Israel moved in as far as the Litani River
and occupied a ten kilometer (six mile) wide corridor north of
its border with Lebanon. About 1,500 Lebanese and
Palestinian civilians were killed in the operation. Some
Israeli forces were withdrawn, but not before the area
was handed over to Israeli allied Christian militiamen
opposed to the Palestinians and to other Arab
Muslim Lebanese.
And then:
Israeli reprisals (spring, 1981)
Ceasefire (July, 1981)
End ceasefire (April, 1982)
Christian militia involvement in Hama, Syria (1982)
Israel invades (1982)
Origins Hizbullah (1982)
Massacres at Sabra and Shatilla (Sept., 1982)
Seeds of Intifada (1983)
U.S. embassy bombed (April, 1983)
Shia-Christian fighting and U.S. intervention (summer, fall, 1983)
From Kerr assassination through Marine barracks bombing (1984)
Israeli pullback (1985)
Fighting (spring, 1985)
Shia-PLO conflict (1987)
General Aoun appointed (Sept., 1988)
Druze shelling of Beirut (1989)
Christian-Muslim fighting (August, 1989)
Ceasefire (September 22, 1989)
Taif Accord (October 1,1989).
http://www.nmhschool.org/tthornton/mehistorydatabase/civil_war_in_lebanon.htm